Mercury levels have tripled in many parts of the ocean, and are highest around Iceland and Antarctica

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But the levels are much lower than expected, Lamborg says, given known emissions from coal burning, cement production, waste incineration and small-scale gold mining. He estimates the oceans contain between 60,000 and 80,000 tonnes of mercury, less than a quarter of the 350,000 tonnes expected (Environmental Science & Technology, doi.org/ckm949).

Where is the rest? Small-scale gold mining may be a big source, so the lost mercury could be in soils near mines, Lamborg says.

Alternatively, the lost mercury could be in sediments of estuaries and coastal waters, particularly in Asia. Last month, Helen Amos of Harvard University estimated that up to 90 per cent of the mercury flowing down rivers from mining areas ends up in these sediments (Environmental Science & Technology, doi.org/t2h).

If those sediments get stirred up, local mercury levels could reach those seen at Minamata, which affected thousands of people.